1. Field of the Invention
This invention relates to pumping apparatus for transporting fluids from a well formation to the earth""s surface. More particularly, the invention pertains to a double-acting, reciprocating downhole pump.
2. Description of the Related Art
Many hydrocarbon wells are unable to produce at commercially viable levels without assistance in lifting formation fluids to the earth""s surface. In some instances, high fluid viscosity inhibits fluid flow to the surface. More commonly, formation pressure is inadequate to drive fluids upward in the wellbore. In the case of deeper wells, extraordinary hydrostatic head acts downwardly against the formation, thereby inhibiting the unassisted flow of fluid to the surface.
A common approach for urging production fluids to the surface includes the use of a mechanically actuated, positive displacement pump. Mechanically actuated pumps are sometimes referred to as xe2x80x9csucker rodxe2x80x9d pumps. The reason is that reciprocal movement of the pump necessary for positive displacement is induced through reciprocal movement of a string of sucker rods above the pump from the surface.
A sucker rod pumping installation consists of a positive displacement pump disposed within the lower portion of the production tubing. The installation includes a piston which is moved in linear translation within the tubing by means of steel or fiberglass rods. Linear movement of the sucker rods is imparted from the surface by a rocker-type structure. The rocker-type structure serves to alternately raise and lower the sucker rods, thereby imparting reciprocating movement to the piston within the pump downhole.
Certain difficulties are experienced in connection with the use of sucker rods. The primary problem is rooted in the fact that most wells are not truly straight, but tend to deviate in various directions en route to the zone of production. This is particularly true with respect to wells which are directionally drilled. In this instance, deviation is intentional. Deviations in the direction of a downhole well cause friction to occur between the sucker rod and the production tubing. This, in turn, causes wear on the sucker rod and the tubing, necessitating the costly replacement of one or both. Further, the friction between the sucker rod and the tubing wastes energy and requires the use of higher capacity motors at the surface.
In an attempt to overcome this problem, submersible electrical pumps have been developed. These pumps are installed into the well itself, typically at the lower end of the production tubing. State of the art submersible electrical pumps comprise a cylindrical assembly which resides at the base of the production string. The pump includes a rotary electric motor which turns turbines at a high horsepower. These turbines are placed below the producing zone of a well and act as fans for forcing production fluids upward through the production tubing.
Efforts have been made to develop a linear electric motor for use downhole. One example is U.S. Pat. No. 5,252,043, issued to Bolding, et al., entitled xe2x80x9cLinear Motor-Pump Assembly and Method of Using Same.xe2x80x9d Other examples include U.S. Pat. No. 4,687,054, issued in 1987 to Russell et al. entitled xe2x80x9cLinear Electric Motor For Downhole Use,xe2x80x9d and U.S. Pat. No. 5,620,048, issued in 1997, and entitled xe2x80x9cOil-Well Installation Fitted With A Bottom-Well Electric Pump.xe2x80x9d In these examples, the pump includes a linear electric motor having a series of windings which act upon an armature. The pump is powered by a cable extending from the surface to the bottom of the well, and residing in the annular space between the tubing and the casing. The power supply generates a magnetic field within the coils which, in turn, imparts an oscillating force upon the armature. In the case of a linear electric motor, the armature would be translated in an up-and-down fashion within the well. The armature, in turn, imparts translational movement to a piston, or connector shaft, residing below the motor. The linear electric motor thus enables the piston of a positive displacement pump to reciprocate vertically, thereby enabling fluids to be lifted with each stroke of the piston.
Submersible pump assemblies which utilize a linear electric motor have not been introduced to the oil field in commercially significant quantities. Such pumps would suffer from several challenges, if employed. One such relates to the volume of fluids which can be lifted with each stroke. In this respect, the typical positive displacement pump will only capture fluids on either the upstroke or the downstroke, depending on its design. Most commonly, fluids are captured, or xe2x80x9cgulped,xe2x80x9d on the downstroke, with the captured volume of fluid flowing through a pump outlet at the top of the pump and then being lifted on the upstroke. Therefore, current positive displacement pumps are considered single acting, and not double-acting. Stated another way, fluid is only captured during a single phase of the stroke, and not during both phases of the stroke.
One obstacle encountered with the design of pumps pertains to hydrostatic balancing. In order to maximize efficiency of a motor apparatus for reciprocating a downhole pump, it is desirable that the pump be hydrostatically balanced. This means that the force required to move the pumping chamber on the upstroke is essentially the same as that required to move the pumping chamber back down on the down stroke. In the typical rocker-beam type lifting arrangement, the downhole pump is biased downward due to the action of hydrostatic head against the pump. Thus, the motor employed for lifting fluids via reciprocation of sucker rods requires that the motor have the capacity to lift a full column of fluid on the upstroke. The pump then simply falls back down on the downstroke in response to the weight of the sucker rods. Therefore, a linear electrical pump design which provides for hydrostatic balancing is desirable so that the force of the pump acting upward is used to displace fluids rather than to purely overcome the hydrostatic pressure differential.
In view of the above discussion, it is apparent that a more effective positive displacement pump is needed in order to transport formation fluids through the production tubing and to the earth""s surface. In addition, a reciprocating pump is needed which is double-acting, that is, it is able to displace fluids both on the down stroke and on the upstroke. Further, a downhole pump is needed which permits the capture of a greater volume of fluids without a corresponding increase in velocity of the fluids through the pump. Further still, a linear pump is needed that is substantially hydrostatically balanced.
A positive displacement pump for pumping fluids from a downhole formation to the earth""s surface is provided. The pump first comprises a hollow plunger. The plunger is reciprocated axially within the wellbore by a linear actuator, such as a submersible linear electric motor, in order to form an upstroke and a downstroke. A pump inlet is disposed at the bottom end of the plunger, while a pump outlet is disposed at the top end of the plunger. The pump is configured such that it is able to pump a first volume of fluid upward within the production tubing during the pump""s upstroke, and a second volume of fluid upward within the tubing during the pump""s downstroke. Thus, the pump is xe2x80x9cdouble-acting.xe2x80x9d
In one embodiment, the piston resides within a tubular housing. A piston is positioned in the annular region between the hollow plunger and the housing. The piston is connected to the plunger, and moves up and down with the plunger. Upper and lower housing heads are also placed in the housing annulus, with the upper housing head fixedly residing above the piston, and the lower housing head fixedly residing below the piston. One or more ports are provided in the piston between the plunger and the lower housing head.
On the upstroke of the plunger, formation fluids are drawn (1) through the inlet port, (2) into the bore of the plunger, and (3) into the housing annulus below the piston. On the downstroke, formation fluids are (1) expelled from the housing annulus, (2) up through the outlet port, and (3) up the production tubing towards the surface. Thus, the pump is able to positively displace formation fluids on both the up stroke and the down stroke of the pump.
A second, alternative embodiment for a double-acting pump is also provided. In the second embodiment, the same inlet and outlet configurations are utilized, and the same seal configurations are used. However, in the second embodiment, a sleeve is nested between the plunger and the housing. Thus, a separate sleeve annulus and housing annulus are created.
In the second embodiment, a through-opening is also provided through the sleeve between the upper sleeve head and the piston. In this manner, fluid communication is attained between the housing annulus and the sleeve annulus. A second pump inlet and pump outlet are also provided in the housing annulus to define a second path of fluid flow. Thus, two possible flow paths for production fluids are providedxe2x80x94one through the plunger, and one through the housing annulus.
In the second embodiment, the upper sleeve annulus is pressurized during the upstroke, and fluid is pumped through both the sleeve through-opening and through the check valve at the second pump outlet. While the upper sleeve annulus is pumping, the lower sleeve annulus is depressurized to inlet pressure. As its volume increases, it pulls a relative vacuum and fills with fluid. Fluid enters through the inlet check valve at the lower end of the plunger. During the downstroke, the lower sleeve annulus pressurizes and fluid flows out of the lower sleeve annulus and up through the check valve at the first outlet, located at the upper end of the plunger. The check valve at the lower end of the plunger is forced to its closed position during this portion of the pumping cycle. At the same time, the second check valve at the upper portion of the housing annulus also closes, and the upper sleeve annulus increases in volume and draws fluid in through the second inlet at the lower end of the housing annulus. In this manner, the lower sleeve annulus is pumping and the upper sleeve annulus is filling during a first phase pump cycle, and they reverse roles during the second phase of the pump cycle.